1. Field of the Invention
The present invention relates to fastener systems and, in particular, to locking fastener systems.
2. Description of Related Art
Various locking fastener systems have been developed to join together materials under a desired compressive force and to "lock" in place to prevent unintentional loosening primarily due to vibrational forces. Prior fastener systems commonly include a collar (i.e., a nut) which fastens to a fastener (i.e., a bolt or a pin) to produce a predetermined preload between the fastener and the collar. That is, a tension force develops between the fastener and the collar as the collar is threaded onto the fastener, placing the intervening joint in compression. The collar subsequently locks onto the fastener by a variety of means.
Some prior locking fastener systems rely on swaging the collar onto the fastener to lock the components together. Examples of locking fastener systems which swag collar material into the fastener are disclosed in U.S. Pat. No. 4,383,353, issued to Stencel; U.S. Pat. No. 4,601,623, issued to Wallace; and U.S. Pat. No. 5,145,300, issued to Wallace.
These prior fastener systems, however, suffer from several drawbacks. Prior fastener systems which first generate a preload and subsequently swage collar material into the fastener tend to produce scattered preload values. That is, the designed preload value produced by a particular size of fastener could widely ranged between a minimum value and a maximum value. For instance, a 1/4"-28 UNF titanium fastener system, of the type disclosed in application Ser. No. 07/481,373, typically produces preload values ranging between 1500 lbs. and 3000 lbs. Consequently, industry commonly used larger fastener systems to ensure a minimum preload value; however, larger fastener systems increase the weight of the assembly, which the aeronautical and aerospace industries particularly disfavor.
Additionally, prior installation tools used with these locking fastener systems further exacerbate the scattering of preload values. Prior installation tools tend to prematurely disengage from the collar (i.e., "cam-off"). That is, the forward end of the collar resists being deformed and forces the installation tool away from the collar. The resultant force, in combination with the continued rotation of installation tool, winds the installation tool off of the collar prior to completely producing the desired preload, thereby frustrating the installation process and generating less compression in the intervening joint than desired.
Scattered preload values also result from a technician's efforts to keep the installation tool on the collar (i.e., to compensate for the cam-off tendency) by applying end pressure to the installation tool which varies from one installation to another. And, although the massive end pressure exerted by the technician may prevent the installation tool from camming off, a corresponding exertion of equal effort may be required to extract the installation tool after installation, further frustrating the installation process.
Moreover, prior installation tools require about 360.degree. of tool rotation about the collar, once the desired preload has been produced, in order to swage the collar onto the fastener. In confined working quarters, this degree of tool rotation is difficult, even using a ratcheting wrench.
Finally, prior installation tools and fastener systems tend to over-strain a flimsy hexagon key which engages a cooperative recess in the tail of the fastener and carries the reaction force during blind side installation. The hexagon key commonly breaks under excessive force or the fastener tail recess commonly erodes. Over straining results when the swaging operation commences while the collar is still capable of rotating under the applied torque.